1. Field of the Invention
The invention relates to devices containing organic semiconductor materials, in particular thin film transistors containing such materials.
2. Discussion of the Related Art
Organic thin film transistors (TFTs) are expected to become key components of the plastic circuitry in, among other things, display drivers of portable computers and pagers, and memory elements of transaction cards and identification tags, where ease of fabrication, mechanical flexibility, and moderate operating temperatures are important considerations. A typical organic TFT is shown in FIG. 1. The TFT contains a source electrode 10, a drain electrode 12, a gate electrode 14, a gate dielectric 16, a substrate 18, and the semiconductor material 20. When the TFT operates in an accumulation mode, the charges injected from the source 10 into the semiconductor are mobile and conduct the source-drain channel current, mainly in a thin channel region within about 100 Angstroms of the semiconductor-dielectric interface. (See, e.g., M. A. Alam et al., "A Two-Dimensional Simulation of Organic Transistors," IEEE Transactions on Electron Devices, Vol. 44, No. 8 (1997).) In the configuration of FIG. 1, the charge need only be injected laterally from the source 10 to form the channel. In the absence of a gate field, the channel ideally has few charge carriers, and there is ideally no source-drain conduction. The off current is defined as the current flowing between the source 10 and the drain 12 when charge has not been intentionally injected into the channel by the application of a gate voltage, and for an accumulation mode TFT, this occurs for a gate-source voltage more positive (for p-channel) or negative (for n-channel) than a certain voltage known as the threshold voltage. (See, e.g., S. M. Sze, Semiconductor Devices--Physics and Technology, John Wiley & Sons (1985).) The on current is defined as the current flowing between the source 10 and the drain 12 when the channel is conducting. For a p-channel accumulation-mode TFT, this occurs at a gate-source voltage more negative than the threshold voltage, and for an n-channel accumulation mode TFT, this occurs at gate-source voltage more positive than the threshold voltage. It is desirable for this threshold voltage to be zero, or slightly positive, for n-channel operation. Switching between on and off is accomplished by the application and removal of an electric field from the gate electrode 14 across the gate dielectric 16 to the semiconductor-dielectric interface, effectively charging a capacitor.
Organic semiconductors provide the switching and/or logic elements in such TFTs. Significant progress has been made in the development of these semiconductors, with mobilities well above 0.01 cm.sup.2 /Vs and on/off ratios greater than 1000 demonstrated for several classes of compounds, including compounds capable of operation in air. With these properties, TFTs are capable of use for applications such as pixel drivers for displays and identification tags. However, most of the compounds exhibiting these desirable properties are p-type, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device.
Yet, one important type of TFT circuit, known as a complementary circuit, desirably contains an n-type semiconductor material exhibiting desirable properties. (See, e.g., A. Dodabalapur et al., "Complementary circuits with organic transistors," Appl. Phys. Lett., Vol. 69, No. 27, 4227 (1996).) The fabrication of complementary circuits requires at least one p-channel TFT and at least one n-channel TFT (n-channel indicating that positive gate voltages, relative to the source voltage, are applied to induce negative charges into the channel region of the device). In particular, simple components such as inverters have been realized using complementary circuit architecture. Advantages of complementary circuits, relative to ordinary TFT circuits, include lower power dissipation, longer lifetime, and better tolerance of noise. It is often desirable to have the mobility and on/off ratio of an n-channel device be of similar magnitude to the mobility and on/off ratio of a p-channel device. Hybrid complementary circuits using an inorganic n-channel semiconductor are known, as reflected in A. Dodabalapur et al., Appl. Phys. Lett., Vol. 68, 2264 (1996), but for ease of fabrication, an organic n-channel semiconductor material is desired.
Only a limited number of materials have been developed for the n-type component of such organic complementary circuits, however. Specifically, buckminsterfullerene (C.sub.60) exhibits a mobility of 0.08 cm.sup.2 /Vs but is unstable in air. Perfluorinated copper phthalocyanine has a lower mobility, about 0.03 cm.sup.2 /Vs, but is generally stable to air operation. Other n-channel semiconductors, including some based on naphthalene frameworks, have also been reported, but with lower mobilities. (See, e.g., J. G. Laquindanum et al., "n-Channel Organic Transistor Materials Based on Naphthalene Frameworks," J. Am. Chem. Soc., Vol. 118, 11331 (1996).) One such naphthalene-based n-channel semiconductor, tetracyanonaphthoquinodimethane (TCNNQD), is capable of operation in air, but the material has displayed a low on/off ratio and is also difficult to prepare and purify. Moreover, there have been no n-channel organic materials capable of being deposited onto a substrate from solution, e.g., as opposed to sublimation, and many organic n-channel materials are actually highly insoluble or unstable to dissolution. In addition, the high-mobility (&gt;0.01 cm.sup.2 /Vs) compounds previously reported are highly absorbing in the visible region of the spectrum.
Due to the advantages offered by complementary TFT circuits, improved organic n-channel materials are desired, in particular organic n-channel materials exhibiting high performance, easy processability, and stability in air, and, advantageously, also transparency to visible light.